In-game, Almost anything else could work, but in my opinion these'd fit well with your team:
Vileplume / Bellossom / Roserade
Your team is lacking a Grass Type, And these could fit well, while Vileplume and Roserade give you a double-psychic weakness, It doesn't matter too much. They all have relatively decent Special Attack stats, So they can be used offensively, and if you have Mega Charizard Y, you could make use of Chlorophyll on Vileplume / Bellossom.
Garchomp / Salamence
Your team also lacks a Dragon Type, which is a good typing. Garchomp and Salamence are both good offensive Dragon types, and they aren't found that late in the game. Dragon is a good typing to have because they're generally offense-based Pokémon, and they have a ton of resistances which makes it harder to KO them.
Aegislash / Scizor
Both of these Pokémon would make good tanks. Scizor can be semi-speedy but Aegislash could be a mixed attacker. I'm mainly suggesting these because your team lacks a Steel Type, which is a great defensive Typing. If you don't have anyone to trade with, or you don't want to wait until almost the end of the game, Aegislash is your other option.
Pokémon Locations: Oddish and Honedge are found on Route 6 (Palais Lane), Roselia is found on Route 7 (Rivière Walk), Gible is found on Route 13 (Lumiose Badlands), Scyther is found on Route 21 (Dernière Way) And Bagon is found on Route 8 (Muraille Coast).
If you'd like Movesets for any of these, just ask. I can probably try to provide a good in-game set.
Hope I helped!~